Use immutable struct where possible

Author: jmert

src/HDF5.jl

@@ -284,7 +284,7 @@ end
 convert(::Type{hid_t}, attr::HDF5Attribute) = attr.id
 show(io::IO, attr::HDF5Attribute) = isvalid(attr) ? print(io, "HDF5 attribute: ", name(attr)) : print(io, "HDF5 attribute (invalid)")
 
-mutable struct HDF5Attributes
+struct HDF5Attributes
     parent::Union{HDF5File, HDF5Group, HDF5Dataset}
 end
 attrs(p::Union{HDF5File, HDF5Group, HDF5Dataset}) = HDF5Attributes(p)